Historically, high purity CF4, which is used to etch silica in the manufacture of integrated circuits, has been derived by the direct fluorination of carbon. Purification of the CF4 gas formed, which includes the removal of impurities such as C2F6, has been effected by temperature swing adsorption using a zeolite bed, such as, NaX (13X). CF4 is co-adsorbed with the C2F6 impurity on the 13X zeolite. When the bed is saturated with C2F6, a N2 purge is passed through the spent adsorbent bed in order to recover some of the coadsorbed CF4 from the bed. The CF4, which is less strongly adsorbed than the C2F6, desorbs first into the N2 purge gas and then the C2F6 is desorbed. The N2 purge gas containing desorbed CF4, and small amounts of desorbed C2F6, is passed through a second bed of 13X zeolite until the C2F6 concentration in the effluent rises to an unacceptable level in the CF4/N2 mixture. By this process about half of the co-adsorbed CF4 can be recovered before the C2F6 concentration in the effluent becomes too high.
Representative patents relating to the separation of carbon fluoride gases are as follows:
U.S. Pat. No. 6,187,077 discloses a process for separating at least one of CF4 and C2F6 from a gas containing at least one of NF3, CHF3 and N2 and SF6. The process steps include 1) passing a feed stream containing various impurities through a glassy membrane to produce a retentate stream rich in SF6 and at least one of CF4 and C2F6, and, then 2) contacting the retentate stream with an adsorbent effective to adsorb SF6 and produce a product stream rich in at least one of CF4 and C2F6. Representative adsorbents include zeolites, preferably X types, activated carbons, e.g., BPL, (data sheet indicates a CCl4 activity of 60-65), PCB (data sheet indicates a CCl4 activity of 60), BAC, F-300, F-400, BPL, RB2 (data sheet indicates an activity of 65) with PCB being the preferred activated carbon. Polymeric adsorbent resins, and carbon molecular sieves are also disclosed.
U.S. Pat. No. 5,523,499 discloses a process for the purification of C2F6 contaminated with CClF3 and CHF3 impurities by adsorption. The C2F6 gas contaminated with impurities is contacted with a sorbent which includes zeolite molecular sieves and activated carbons. Preferred activated carbons, such as BPL from the Calgon Corporation and Type UU from Barneby and Sutcliffe Corp having a particle size of from 4 to 325 mesh.
Japanese Patent Application No. 54-62867 (disclosure 55-154925) discloses a purification process for CF4 containing CF3Cl as an impurity. The process comprises the steps of irradiating the gas stream with a laser and absorbing photons in the fluorine compounds thereby converting the CF3Cl to C2F6 and Cl2. The C2F6 is then removed via distillation or adsorption.
There is a need in the industry for adsorbents that would allow for a long onstream time for a given column size and a need for adsorbents which have a higher selectivity for carbon fluoride impurities other than the CF4 product. Such improved adsorbents would enhance the recovery of CF4.
This invention relates to an improvement in a process for removing C2F6 as an impurity from a CF4 containing gas, preferably CF4 produced by the reaction of F2 with carbon. The improvement in the process comprises the steps:
contacting said CF4 containing gas with an activated carbon having a CCl4 activity from 43 to 55 in an adsorption bed to effect selective adsorption of said C2F6 impurity; and,
recovering a purified CF4 product in the effluent from said adsorbent bed.
Significant advantages of this process include:
an ability to remove impurities from a CF4 containing gas stream contaminated with fluorocarbon impurities;
an ability to selectively adsorb contaminant C2F6 without effecting substantial losses to irreversible adsorption of CF4;
an ability to provide for long onstream times in the adsorption process; and,
an ability to achieve effective removal of the fluorocarbon impurities from the CF4 product.